1. An unpleasant surprise.
2. When a woman hasn't shaven her vagina in months and the guy pulls out and ejaculates on top of her vagina.
"Ahoolah hoolah hoolah, aboolah boolah boolah! Look who's got front row seats to the Mexican hat-dance now, just like a bunch of spiders in a birthday cake!"
"He was surprised at the sight of her unshaven vagina as his ejaculate got stuck in her pubic hair. Just like a bunch of spiders in a birthday cake."